This invention relates in general to prime movers and, in particular, this invention relates to an oil supply system for a prime mover wherein the prime mover requires both high pressure oil and low pressure oil.
One type of prime mover is a steam turbine wherein an oil supply system is required to provide hydraulic fluid for control purposes and lubrication purposes. High pressure control oil is needed for positioning steam inlet valves whereas low pressure oil is needed for lubricating tubine bearings. In addition, low pressure oil may be used for positioning pilot valves which supply fluid logic in modern steam turbine control systems.
Prior art oil supply systems include an oil pump which may be driven by the main turbine shaft through a gear coupling. The pump was designed to provide a single oil output at the highest overall required pressure which, as previously mentioned, is the control oil for steam inlet valve positioning. Lubrication and other low pressure oil requirements were satisfied by low pressure oil lines which included pressure reducing valves. This system, as described, has several disadvantages in that all the oil which is supplied is raised to the higher energy level requiring a larger pump motor and greater consumption of energy. Moreover, the oil runs hotter at a higher pressure requiring increased oil cooler duty. Another difficulty inherent in the afore-described system occurs during transfer from one main oil pump to a stand-by main oil pump where upon as the system is readjusting to proportionate flow, the low pressure requirement may override the high pressure requirement causing the steam inlet valve to move to a more closed position. Other systems are known wherein two pumps are utilized, one pump for high pressure and the other pump for low pressure. However, one disadvantage of this type of system is that if the low pressure system fails while the high pressure system causes the valves to remain open, damage could occur to system components which require lubrication.
The present invention seeks to overcome the foregoing difficulties and deficiencies according to the following brief summary of the invention. A centrifugal dual pressure level oil pump provides a high pressure oil discharge for the control portion of the turbine and a low pressure oil discharge for lubrication. Hence, the need for pressure reducing valves in the low pressure oil supply pipes is obviated and pump horsepower is conserved since only a portion of the total pump output is raised to the higher pressure. Moreover, a pump failure will automatically close the steam inlet valves in order to protect the turbine bearings. The centrifugal pump is a multi-stage device sized so that the low pressure discharge is taken downstream from a first stage and the high pressure discharge is taken downstream from the last stage. The pump has a lower horsepower rating and therefore is less expensive to operate. The low pressure oil runs cooler and during pump transfer to a stand-by pump, pressure differential valves prevent the loss of control oil to lubrication oil while the stand-by pump is coming up to speed. Hence the position of the steam valves does not change due to oil pressure decay.
It is therefore one object of the present invention to provide an oil supply system for a turbomachine having improved efficiency.
It is another object of the present invention to provide an oil supply system for a turbomachine which obviates losses associated with the use of pressure reducing valves.
It is a further object of the invention to provide an oil supply system for a turbomachine which may avoid sacrificing high pressure oil in order to satisfy low pressure oil during pump transfer conditions.
The foregoing is but a brief introduction into the objects, advantages and operation of the present invention. Other objects and advantages will become apparent from the following detailed description of the invention and the novel features will be particularly pointed out herein after in the claims.